This invention relates to a method for sensing the volume flow rate of blood and similar liquids, and especially to an improved and more convenient measurement technique using a sector scanning steered beam imager having a Doppler mode of operation.
Volume blood flow information is important to the diagnosis of cardiovascular disease. Knowledge of the amount of oxygen being delivered to various regions of the body is a factor that can assist clinicians in the management of disease and trauma. Volume blood flow may be one of the best indicators of available oxygen. It is also an indication of the ability of the heart to function as a pump to maintain normal body processes. In the case of a vessel, volume flow rate is given by the equation Q=V.multidot.A, where A is the cross-sectional area normal to the vessel center line and V is the velocity of blood passing through the cross-sectional area and at right angles to it. In the circulatory system A and V are not only functions of position, but also of time.
Various methods have been proposed to estimate volume blood flow rate by ultrasound. Most involve making assumptions about the flow conditions and the velocity profile across the vessel lumen. In most, orientation is attempted without imaging help and insonification of the sample volume is performed "blind." Several available flowmeters employ the zero-crossing detection method which gives only a crude estimate of root-mean-square velocity. For narrow Doppler spectra the error in the calculation is not catastrophic. In large vessels such as the abdominal aorta, or in situations involving complex velocity distributions such as at the root of the ascending aorta, the Doppler spectra may be quite broad and the discrepancy unacceptable. An advanced Doppler blood flow instrument is reported by C. F. Hottinger and J. D. Meindl in Proceedings of the IEEE, Vol. 63, No. 6, June 1975, pp. 984,5. This employs a double aperture configuration to illuminate the lumen and estimate its cross-sectional area. It is stated that by electronically forming a ratio of the Doppler signal power returning to the two apertures, orientation ambiguities are removed. U.S. Pat. No. 3,939,707 to G. Kossoff describes the combined use of B-scan imaging and a Doppler technique. The cross-sectional area is determined by measuring the vessel diameter on the display, and the angle between the ultrasonic beam direction for the Doppler measurement and the vessel axis is also estimated and is factored in the velocity reading. This method can lead to errors large enough that the data has little or impaired diagnostic value.
In addition to the problem of correct orientation of the ultrasonic beam with respect to the flow vectors, another difficulty in estimating volume blood flow by transcutaneous ultrasonic transducers arises from the need to define accurately a sample volume which spans the entire vessel cross section. Finally, it is important to process the ultrasound signals echoing from the vessel lumen by a technique that faithfully yields mean velocity readings.